I was searching for alternatives to default Reader Mode of Firefox browser because on forum Sufficient Velocity it works in a really wonky way making it completely unusable. There is some weird interaction with "Threadmark" function of Sufficient Velocity forum that causes (Default) Reader Mode to fetch text from always the wrong post (either last or the next one).
This addon solves that problem thanks to option to manually direct the source of fetched data through highlighting the correct post bypassing the weird interactions. Exactly what I was looking for!